In general, artificial light sources are available in either a predetermined broad spectrum or in an almost monochromatic form. Some well known "white" light sources include incandescent lamps, high intensity discharge (HID) light sources and fluorescent light sources. Among the better known monochromatic light sources we can cite light emitting diodes, lasers of all types and gas discharge tubes. What has not been freely available heretofore, is an efficient light source with a temporally variable spectral output, or at least a time-variable appearance. Currently, when the need for such means of illumination arises, one can either use filters to allow only part of the spectrum of a white light source to be seen, or, one can use discrete light sources of different spectral distribution to illuminate a single scene. Both of these approaches are cumbersome.
For instance, when using filters, the segment of the spectrum filtered out is lost to absorption and thus there is a major reduction in the system efficiency. It is desirable to have flat light sources with variable chromaticity and a large degree of luminance homogeneity, which can be modular so as to allow for stepwise increase in total light output from single powering source.
In most current lighting systems, the conversion of electrical power to light flux occurs at the point of use of the light. Since the efficiency of conversion of electricity to visible light is rarely more than 25%, and in some very large light sources, 30%, a large amount of heat must be dissipated at the point of use of the light. Furthermore, there are special situations, particularly in hazardous environments, where it is not desirable to have electrical connectors, conductors or any electrically powered devices within the space that is to be illuminated. In such areas the light sources and their associated wiring must be enclosed in special enclosures, a fact that increases installation costs and reduces efficiency of light output utility. It is thus desirable to provide luminaires which are remotely powered so as to allow for heat withdrawal at the remote location where electricity is converted to light, allow safe installation in hazardous environment, and provide for controllable chromaticity.
Light sources of the latter type can be used in a broad variety of environments including photographic dark rooms, semiconductor and other clean room manufacturing environments as well as in chemical plants where explosive or flammable products are being manufactured.